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Cite this: DOI: 10.1039/c4cc09207d Received 18th November 2014, Accepted 27th February 2015
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A Si–MnOOH composite with superior lithium storage properties† Hai Zhong,ab Yanbo Yang,a Fei Ding,b Donghai Wang,c Yunhong Zhoua and Hui Zhan*a
DOI: 10.1039/c4cc09207d www.rsc.org/chemcomm
The Si–MnOOH composite electrode exhibits very stable cycling and excellent rate capability, such as 1200 mA h g 700 mA h g
1
1
at 12 A g 1, and
at 20 A g 1. The c-MnOOH component significantly
promotes the alloying/de-alloying reaction between Si and lithium.
High energy/power density Li-ion batteries have been pursued since the commercialization of graphite/LiCoO2 LiBs; among all the cathode/anode combinations, LiBs with Si anodes attract more attention due to their potential high energy density. Nowadays, silicon triggers much efforts due to its largest theoretical capacity (Li22Si5, B4200 mA h g 1), and relatively lower electrochemical alloy/de-alloy reaction voltage (B0.4 V vs. Li/Li+).1 However, the repeated large volume changes during Li insertion and extraction can lead to electrode failure.2 The failure mechanism is believed to involve particle pulverization, and electric contact deterioration which will finally cause electrode disintegration and rapid capacity fading.3 Recent research also proves that the solid–electrolyte interphase (SEI) degradation is another reason for the Si anode failure.4,5 A number of strategies have been implemented to suppress the volume change and enhance or maintain the electric contact between silicon and the conductive agent. Examples typically are Si or Si–C composites with specially designed architecture, such as silicon nanowires, silicon nanorods, porous silicon and york-shell silicon–C composites,6–9 and they exhibit the expected capacity performance and cycling stability. However, the relatively complicated multi-step synthesis and very fluffy, insufficiently compressible product powder bring concerns on cost and the electrode fabrication. Other than the special material processing or special architecture design, binder optimization or SEI additive has also been proposed to enhance a
Department of Chemistry, Wuhan University, Hubei, 430072, China. E-mail: [email protected] b Tianjin Institute of Power Source, Tianjin, 300381, China c Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania, 16802, USA † Electronic supplementary information (ESI) available: Sample synthesis, experimental conditions, electrochemical properties of MnOOH and different Si/ MnOOH ratios. See DOI: 10.1039/c4cc09207d
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the properties of the Si anode, and carboxymethylcellulose (CMC) or alginate binder and fluoro-ethylene carbonate (FEC) additives are now widely used in the research on the Si anode.10–12 Though many composites with good properties have been successfully synthesized, most of them use carbon or conductive polymers to enhance the electrical contact or buffer the volume expansion. Here we report a new Si–MnOOH composite which shows greatly improved Li-ion storage performance. Hydrothermally prepared g-MnOOH13,14 was ball milled with silicon to form a Si–MnOOH composite. The obtained Si–MnOOH composite exhibits an initial charge capacity of 3058 mA h g 1 at a current of 0.1 A g 1 between 1.5 and 0.01 V. An excellent rate capability has been achieved by the composite electrode, and it shows a capacity of 1200 mA h g 1 at 12 A g 1 and 700 mA h g 1 at 20 A g 1. In addition, the Si–MnOOH composite presents a very stable cycling with more than 1000 mA h g 1 after 1500 cycles. The superior electro-activity toward lithium of the Si–MnOOH composite should be attributed to g-MnOOH which has some ‘‘catalyzing effect’’ on the alloying/de-alloying reaction between lithium and silicon.15 The XRD pattern of the hydrothermally synthesized g-MnOOH is shown in Fig. 1a. It well agreed with the literature data (manganite, space group P21/c(14), JCPDS 41-1379) and all of the diffraction peaks could be indexed by a monoclinic phase. The sharp peaks indicated good crystallinity and the very notable (111% ) diffraction line suggested a preferred orientation of the product which could be further evidenced in SEM observation. A scanning electron microscopy (SEM) (Fig. 1b) image revealed that the as-synthesized g-MnOOH was one-dimensional (1D) nanomaterial, and was composed of the nanorods with an average diameter of 40 nm. The SEM images of silicon and the composite powder are shown in Fig. 1c and d respectively. Bare Si has spherical particles with a diameter of about 100 nm, and slight particle agglomeration could be observed. After the high-energy ball-milling with g-MnOOH, the spherical morphology was mostly reserved in Si–MnOOH and the average particle size did not change much. To better analyze the electrochemical activity of the Si–MnOOH composite, g-MnOOH alone was first examined in
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Fig. 1 (a) XRD pattern of g-MnOOH; (b) SEM image of g-MnOOH synthesized by the hydrothermal method; (c) SEM image of the silicon powder; (d) SEM image of the Si–MnOOH composite.
Fig. 2 (a) Plot of charge/discharge capacity and coulombic efficiency vs. cycles of the Si–MnOOH electrode (0.1 A g 1 in the first two cycles, 2 A g 1 in the rest of the cycles); (b) voltage profiles of the Si–MnOOH electrode in different cycles; (c) dQ/dV curve of the Si–MnOOH electrode in the 7th cycle.
a 1 M LiPF6/EC + DMC electrolyte. After the low efficiency in the 1st cycle, the coulombic efficiency increased gradually until finally stabilized at around 98% (Fig. S1a in ESI†). In addition, the voltage profile (Fig. S1b in ESI†) and the differential dQ/dV plot (Fig. S1c in ESI†) indicated that after the 1st charge/ discharge, the electrochemical behavior of g-MnOOH was quite similar to that of MnO except a higher reducing voltage.16,17 The result suggests that the redox reaction of g-MnOOH should follow the route g-MnOOH + Li+ + e 2 MnO + LiOH and MnO + 2Li+ + 2e 2 Mn + Li2O, thus the g-MnOOH phase first decomposes into MnO and then a conversion reaction of MnO occurs. The higher reducing voltage of MnOOH than MnO may further imply the ‘‘catalyzing effect’’ of LiOH, and a similar observation has also been made on the Ni(OH)2 anode which showed a higher reducing voltage than NiO.18,19 The effect of g-MnOOH on the Si anode was investigated by electrochemical tests. The Si–MnOOH composite with different g-MnOOH contents was obtained by ball-milling. The optimum Si–MnOOH ratio of 55 : 5 was chosen according to the result in Fig. S2 (in ESI†). Elemental mapping in Fig. S3b–d (in ESI†) showed the uniform distribution of Mn and O along with silicon in the Si–MnOOH composite (Fig. S3a in ESI†), confirming the homogeneous dispersion of g-MnOOH in the Si powder. The electrochemical properties of the Si–MnOOH composite were evaluated by galvanostatic charge–discharge tests, and the capacity is based on the mass of the composite. Fig. 2a displays the cycling stability of the Si–MnOOH electrode. Two ‘‘formation’’ charging/discharging with a low current of 0.1 A g 1 was applied, and then a current of 2 A g 1 was adopted in the subsequent cycles. In the 1st cycle, the discharge and charge capacity was 3907 mA h g 1 and 3058 mA h g 1, respectively, corresponding to a coulombic efficiency of 78.3%. Upon increasing the current to 2 A g 1, the capacity dropped to 2605/2501 mA h g 1 in the 3rd cycle, but recovered to above 3000 mA h g 1 in the 7th cycle. A similar observation was made previously on nano-Si.20 Actually, the formation process to
completely activate the electrode material has been widely reported on the electrode with a mesoporous/microporous structure and is explained by the gradual electrolyte impregnation into the electrode.21,22 After the initial formation cycles, a maximum capacity of 3056/3007 mA h g 1 was achieved in the 7th cycle. A very slow capacity decay occurred in the subsequent cycles; after 100 cycles, the electrode could release near 2200 mA h g 1 capacity. Even after 1500 cycles, the capacity was still above 1200 mA h g 1 (Fig. S4 in ESI†). The voltage profiles of the Si–MnOOH electrode in different cycles are shown in Fig. 2b. We can see that except in the first cycle, the voltage hysteresis between the charge and the discharge plateaus gradually increased with cycling but still remained very small within 100 cycles. To better understand the redox process of the Si–MnOOH electrode, a differential analysis was conducted on the 7th charge/discharge curve and the result is displayed in Fig. 2c. The alloying/de-alloying of lithium with Si led to two pairs of redox peaks at 0.29/0.08 V and 0.48/0.24 V vs. Li/Li+. The result is consistent with the previous report on Si and should be attributed to the phase transformation between different amorphous LixSi.23 The small voltage segregation between the oxidizing and reducing branches indicated the good reaction reversibility of Si–MnOOH. However, we could not unambiguously discern the redox peak of g-MnOOH, and the reasons may be the low proportion of g-MnOOH in the composite electrode or the redox peaks of g-MnOOH being covered by those of silicon. The Si–MnOOH electrode exhibited an excellent rate capability. In Fig. 3a, at a low current of 0.1 A g 1, the capacity of Si–MnOOH was just slightly higher than that of the Si electrode, such as 3100 mA h g 1 to 2900 mA h g 1; however, when the current increased to 1 A g 1, a capacity of more than 2800 mA h g 1 was obtained on Si–MnOOH while barely 2000 mA h g 1 on Si. The capacity difference was more notable when the current increased further. The pristine Si anode almost failed at a current of 8 A g 1, and the Si–MnOOH electrode still could be
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Fig. 3 (a) Discharging capacity vs. cycle number for the bare silicon electrode and the Si–MnOOH electrode at different currents in the voltage range of 0.01–1.5 V; (b) voltage profiles of the bare silicon electrode and the Si–MnOOH electrode at current densities of 0.1, 0.5, 1, 4, 8, 12, 16 and 20 A g 1; (c) AC impedance of the bare silicon electrode and the Si–MnOOH electrode after 100 cycles.
discharged to around 1600 mA h g 1. The voltage–capacity profile in Fig. 3b more clearly shows the capacity/energy advantage of Si–MnOOH over Si. For Si–MnOOH, the capacity only dropped from 3197 mA h g 1 at 0.1 A g 1 to 2800 mA h g 1 at 1 A g 1, while the voltage gap between charging and discharging remained almost constant. Even at a current of 12 A g 1, a decent capacity of around 1200 mA h g 1 was delivered. It is worth noting that the Si–MnOOH electrode presented an appreciable capacity of 700 mA h g 1 at 20 A g 1. To our knowledge, such capacity has seldom been reported on silicon-based materials in the ultrafast charge/discharge mode. Actually, even on a ‘‘super thick’’ Si–MnOOH electrode (mass loading is 3 mg cm 2, the ratio of active material : acetylene black : PAA binder is 6 : 3 : 1, specific area capacity at 1 A g 1 is 4.5 mA h cm 2), the improvement in rate capability is still very significant (Fig. S5 in ESI†), however, the stable cycling will require more work on the binder because of much more serious electrode peeling during electrode fabrication and electrode cycling.
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As the performance is obtained on the Si–MnOOH composite prepared by a simple ball-milling, further enhanced capacity/ energy performance can be reasonably expected if architecture design or component optimization can be involved. The AC impedance comparison provides some clues for the better electrochemical performance of Si–MnOOH than Si (Fig. 3c). After 100 cycles, impedance spectra were recorded on both electrodes. They all showed two semicircles in the high and high-to-medium frequency range, which represents Li+ diffusion through the SEI and charge-transfer reaction, and can be indexed by RSEI and Rct, respectively, according to the equivalent circuit diagram. The low-frequency Warburg slope represents the semi-infinite diffusion of lithium-ion in the electrodes.24 Comparing the impedance diagrams, we can see that the Si–MnOOH electrode has smaller semicircles than the Si electrode in the high-frequency and medium-frequency regions, suggesting the smaller RSEI and Rct for the Si–MnOOH electrode than the Si electrode. The fitted data in Table S1 (in ESI†) indicate that the Rct value of Si–MnOOH is much smaller than that of the Si electrode, and the result reveals much better reaction kinetics of Si–MnOOH than Si. The very close Warburg factor (s) of Si and Si–MnOOH electrodes suggests a similar Li+ diffusion coefficient of these two electrodes,25 and it is reasonable because Si and Si–MnOOH have very similar particle size and essentially the same structure (the very low proportion of MnOOH in the Si–MnOOH composite will not significantly affect the structure of bulk materials). We think that the impedance results explain the excellent cycling stability and rate capability of the Si–MnOOH composite and may imply the ‘‘catalyzing’’ effect of g-MnOOH on the alloying/de-alloying reaction of the Si anode; more work is still in progress on this point. In conclusion, the g-MnOOH nanorod was prepared through a hydrothermal method, and then ball-milled with Si to form the Si–MnOOH composite. The as-prepared Si–MnOOH composite showed a high capacity of about 3900 mA h g 1 and very stable cycling. Additionally, the composite exhibited an excellent rate capability, such as 1200 mA h g 1 at 12 A g 1, and 700 mA h g 1 at 20 A g 1. The result provides a new strategy to enhance the electrochemical performance of the Si anode, and the facile preparation offers a new way to obtain mass-producible Si anode material with perfect Li-ion storage properties. The authors would like to express their sincere thanks to the National Natural Science Foundation of China (No. 21073138, 21273168, 21233004) for the financial support.
Notes and references 1 J. X. Song, M. J. Zhou, R. Yi, T. Xu, M. L. Gordin, D. H. Tang, Z. X. Yu, M. Regula and D. H. Wang, Adv. Funct. Mater., 2014, 24, 5904–5910. 2 J. H. Ryu, J. W. Kim, Y. E. Sung and S. M. Oh, Electrochem. Solid-State Lett., 2004, 7, A306–A309. 3 X. Yang, Z. Wen, X. Xu, Z. Gu and S. Huang, Electrochem. Solid-State Lett., 2007, 10, A52–A55. 4 M. Y. Nie, D. P. Abraham, Y. J. Chen, A. Bose and B. L. Lucht, J. Phys. Chem. C, 2013, 117, 13403–13412. 5 Y. Liu, X. Guo, J. F. Li, Q. L. Lv, T. Y. Ma, W. T. Zhu and X. P. Qiu, J. Mater. Chem. A, 2013, 1, 14075–14079.
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Communication 6 N. Du, X. Fan, J. X. Yu, H. Zhang and D. R. Yang, Electrochem. Commun., 2011, 13, 1443–1446. 7 R. Teki, M. K. Datta, R. Krishnan, T. C. Parker, T. M. Lu, P. N. Kumta and N. Koratkar, Small, 2009, 5, 2236–2242. 8 H. Zhong, H. Zhan and Y. H. Zhou, J. Power Sources, 2014, 262, 10–14. 9 N. Liu, H. Wu, M. T. McDowell, Y. Yao, C. M. Wang and Y. Cui, Nano Lett., 2012, 12, 3315–3321. 10 J. C. Guo and C. S. Wang, Chem. Commun., 2010, 46, 1428–1430. 11 I. Kovalenko, B. Zdyrko, A. Magasinski, B. Hertzberg, Z. Milicev, R. Burtovyy, I. Luzinov and G. Yushin, Science, 2011, 334, 75–79. 12 Y. M. Lin, K. C. Klavetter, P. R. Abel, N. C. Davy, J. L. Snider, A. Heller and C. B. Mullins, Chem. Commun., 2012, 48, 7268–7270. 13 V. M. B. Crisostomo, J. K. Ngala, S. Alia, A. Dobley, C. Morein, C. H. Chen, X. F. Shen and S. L. Sui, Chem. Mater., 2007, 19, 1832–1839. 14 G. Tao, K. Frank and N. Reinhard, Inorg. Chem., 2009, 48, 6242–6250. 15 L. L. Zhang, X. B. Zhang, Z. L. Wang, J. J. Xu, D. Xua and L. M. Wang, Chem. Commun., 2012, 48, 7598–7600.
Chem. Commun.
ChemComm 16 Y. Xia, Z. Xiao, X. Dou, H. Huang, X. H. Lu, R. J. Yan, Y. P. Gan, W. J. Zhu, J. P. Tu, W. K. Zhang and X. Y. Tao, ACS Nano, 2013, 7, 7083–7092. 17 G. L. Xu, Y. F. Xu, H. Sun, F. Fu, X. M. Zheng, L. Huang, J. T. Li, S. H. Yang and S. G. Sun, Chem. Commun., 2012, 48, 8502–8504. 18 S. B. Ni, X. H. Lv, T. Li, X. L. Yang and L. L. Zhang, J. Mater. Chem. A, 2013, 1, 1544–1547. 19 X. H. Huang, J. P. Tu, X. H. Xia, X. L. Wang and J. Y. Xiang, Electrochem. Commun., 2008, 10, 1288–1290. 20 J. C. Guo, A. Sun and C. S. Wang, Electrochem. Commun., 2010, 12, 981–984. 21 L. C. Yang, Q. S. Gao, Y. H. Zhang, Y. Tang and Y. P. Wu, Electrochem. Commun., 2008, 10, 118–122. 22 Y. F. Shi, B. K. Guo, S. A. Corr, Q. H. Shi, Y. S. Hu, K. R. Heier, L. Q. Chen, R. Seshadri and G. D. Stucky, Nano Lett., 2009, 9, 4215–4220. 23 J. Li and J. R. Dahn, J. Electrochem. Soc., 2007, 154, A156–A161. 24 J. Liu and A. Manthiram, Chem. Mater., 2009, 21, 1695–1707. 25 H. Liu, C. Li, H. P. Zhang, L. J. Fu, Y. P. Wu and H. Q. Wu, J. Power Sources, 2006, 159, 717–720.
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Cite this: DOI: 10.1039/c4cc09207d Received 18th November 2014, Accepted 27th February 2015
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A Si–MnOOH composite with superior lithium storage properties† Hai Zhong,ab Yanbo Yang,a Fei Ding,b Donghai Wang,c Yunhong Zhoua and Hui Zhan*a
DOI: 10.1039/c4cc09207d www.rsc.org/chemcomm
The Si–MnOOH composite electrode exhibits very stable cycling and excellent rate capability, such as 1200 mA h g 700 mA h g
1
1
at 12 A g 1, and
at 20 A g 1. The c-MnOOH component significantly
promotes the alloying/de-alloying reaction between Si and lithium.
High energy/power density Li-ion batteries have been pursued since the commercialization of graphite/LiCoO2 LiBs; among all the cathode/anode combinations, LiBs with Si anodes attract more attention due to their potential high energy density. Nowadays, silicon triggers much efforts due to its largest theoretical capacity (Li22Si5, B4200 mA h g 1), and relatively lower electrochemical alloy/de-alloy reaction voltage (B0.4 V vs. Li/Li+).1 However, the repeated large volume changes during Li insertion and extraction can lead to electrode failure.2 The failure mechanism is believed to involve particle pulverization, and electric contact deterioration which will finally cause electrode disintegration and rapid capacity fading.3 Recent research also proves that the solid–electrolyte interphase (SEI) degradation is another reason for the Si anode failure.4,5 A number of strategies have been implemented to suppress the volume change and enhance or maintain the electric contact between silicon and the conductive agent. Examples typically are Si or Si–C composites with specially designed architecture, such as silicon nanowires, silicon nanorods, porous silicon and york-shell silicon–C composites,6–9 and they exhibit the expected capacity performance and cycling stability. However, the relatively complicated multi-step synthesis and very fluffy, insufficiently compressible product powder bring concerns on cost and the electrode fabrication. Other than the special material processing or special architecture design, binder optimization or SEI additive has also been proposed to enhance a
Department of Chemistry, Wuhan University, Hubei, 430072, China. E-mail: [email protected] b Tianjin Institute of Power Source, Tianjin, 300381, China c Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania, 16802, USA † Electronic supplementary information (ESI) available: Sample synthesis, experimental conditions, electrochemical properties of MnOOH and different Si/ MnOOH ratios. See DOI: 10.1039/c4cc09207d
This journal is © The Royal Society of Chemistry 2015
the properties of the Si anode, and carboxymethylcellulose (CMC) or alginate binder and fluoro-ethylene carbonate (FEC) additives are now widely used in the research on the Si anode.10–12 Though many composites with good properties have been successfully synthesized, most of them use carbon or conductive polymers to enhance the electrical contact or buffer the volume expansion. Here we report a new Si–MnOOH composite which shows greatly improved Li-ion storage performance. Hydrothermally prepared g-MnOOH13,14 was ball milled with silicon to form a Si–MnOOH composite. The obtained Si–MnOOH composite exhibits an initial charge capacity of 3058 mA h g 1 at a current of 0.1 A g 1 between 1.5 and 0.01 V. An excellent rate capability has been achieved by the composite electrode, and it shows a capacity of 1200 mA h g 1 at 12 A g 1 and 700 mA h g 1 at 20 A g 1. In addition, the Si–MnOOH composite presents a very stable cycling with more than 1000 mA h g 1 after 1500 cycles. The superior electro-activity toward lithium of the Si–MnOOH composite should be attributed to g-MnOOH which has some ‘‘catalyzing effect’’ on the alloying/de-alloying reaction between lithium and silicon.15 The XRD pattern of the hydrothermally synthesized g-MnOOH is shown in Fig. 1a. It well agreed with the literature data (manganite, space group P21/c(14), JCPDS 41-1379) and all of the diffraction peaks could be indexed by a monoclinic phase. The sharp peaks indicated good crystallinity and the very notable (111% ) diffraction line suggested a preferred orientation of the product which could be further evidenced in SEM observation. A scanning electron microscopy (SEM) (Fig. 1b) image revealed that the as-synthesized g-MnOOH was one-dimensional (1D) nanomaterial, and was composed of the nanorods with an average diameter of 40 nm. The SEM images of silicon and the composite powder are shown in Fig. 1c and d respectively. Bare Si has spherical particles with a diameter of about 100 nm, and slight particle agglomeration could be observed. After the high-energy ball-milling with g-MnOOH, the spherical morphology was mostly reserved in Si–MnOOH and the average particle size did not change much. To better analyze the electrochemical activity of the Si–MnOOH composite, g-MnOOH alone was first examined in
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Fig. 1 (a) XRD pattern of g-MnOOH; (b) SEM image of g-MnOOH synthesized by the hydrothermal method; (c) SEM image of the silicon powder; (d) SEM image of the Si–MnOOH composite.
Fig. 2 (a) Plot of charge/discharge capacity and coulombic efficiency vs. cycles of the Si–MnOOH electrode (0.1 A g 1 in the first two cycles, 2 A g 1 in the rest of the cycles); (b) voltage profiles of the Si–MnOOH electrode in different cycles; (c) dQ/dV curve of the Si–MnOOH electrode in the 7th cycle.
a 1 M LiPF6/EC + DMC electrolyte. After the low efficiency in the 1st cycle, the coulombic efficiency increased gradually until finally stabilized at around 98% (Fig. S1a in ESI†). In addition, the voltage profile (Fig. S1b in ESI†) and the differential dQ/dV plot (Fig. S1c in ESI†) indicated that after the 1st charge/ discharge, the electrochemical behavior of g-MnOOH was quite similar to that of MnO except a higher reducing voltage.16,17 The result suggests that the redox reaction of g-MnOOH should follow the route g-MnOOH + Li+ + e 2 MnO + LiOH and MnO + 2Li+ + 2e 2 Mn + Li2O, thus the g-MnOOH phase first decomposes into MnO and then a conversion reaction of MnO occurs. The higher reducing voltage of MnOOH than MnO may further imply the ‘‘catalyzing effect’’ of LiOH, and a similar observation has also been made on the Ni(OH)2 anode which showed a higher reducing voltage than NiO.18,19 The effect of g-MnOOH on the Si anode was investigated by electrochemical tests. The Si–MnOOH composite with different g-MnOOH contents was obtained by ball-milling. The optimum Si–MnOOH ratio of 55 : 5 was chosen according to the result in Fig. S2 (in ESI†). Elemental mapping in Fig. S3b–d (in ESI†) showed the uniform distribution of Mn and O along with silicon in the Si–MnOOH composite (Fig. S3a in ESI†), confirming the homogeneous dispersion of g-MnOOH in the Si powder. The electrochemical properties of the Si–MnOOH composite were evaluated by galvanostatic charge–discharge tests, and the capacity is based on the mass of the composite. Fig. 2a displays the cycling stability of the Si–MnOOH electrode. Two ‘‘formation’’ charging/discharging with a low current of 0.1 A g 1 was applied, and then a current of 2 A g 1 was adopted in the subsequent cycles. In the 1st cycle, the discharge and charge capacity was 3907 mA h g 1 and 3058 mA h g 1, respectively, corresponding to a coulombic efficiency of 78.3%. Upon increasing the current to 2 A g 1, the capacity dropped to 2605/2501 mA h g 1 in the 3rd cycle, but recovered to above 3000 mA h g 1 in the 7th cycle. A similar observation was made previously on nano-Si.20 Actually, the formation process to
completely activate the electrode material has been widely reported on the electrode with a mesoporous/microporous structure and is explained by the gradual electrolyte impregnation into the electrode.21,22 After the initial formation cycles, a maximum capacity of 3056/3007 mA h g 1 was achieved in the 7th cycle. A very slow capacity decay occurred in the subsequent cycles; after 100 cycles, the electrode could release near 2200 mA h g 1 capacity. Even after 1500 cycles, the capacity was still above 1200 mA h g 1 (Fig. S4 in ESI†). The voltage profiles of the Si–MnOOH electrode in different cycles are shown in Fig. 2b. We can see that except in the first cycle, the voltage hysteresis between the charge and the discharge plateaus gradually increased with cycling but still remained very small within 100 cycles. To better understand the redox process of the Si–MnOOH electrode, a differential analysis was conducted on the 7th charge/discharge curve and the result is displayed in Fig. 2c. The alloying/de-alloying of lithium with Si led to two pairs of redox peaks at 0.29/0.08 V and 0.48/0.24 V vs. Li/Li+. The result is consistent with the previous report on Si and should be attributed to the phase transformation between different amorphous LixSi.23 The small voltage segregation between the oxidizing and reducing branches indicated the good reaction reversibility of Si–MnOOH. However, we could not unambiguously discern the redox peak of g-MnOOH, and the reasons may be the low proportion of g-MnOOH in the composite electrode or the redox peaks of g-MnOOH being covered by those of silicon. The Si–MnOOH electrode exhibited an excellent rate capability. In Fig. 3a, at a low current of 0.1 A g 1, the capacity of Si–MnOOH was just slightly higher than that of the Si electrode, such as 3100 mA h g 1 to 2900 mA h g 1; however, when the current increased to 1 A g 1, a capacity of more than 2800 mA h g 1 was obtained on Si–MnOOH while barely 2000 mA h g 1 on Si. The capacity difference was more notable when the current increased further. The pristine Si anode almost failed at a current of 8 A g 1, and the Si–MnOOH electrode still could be
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Fig. 3 (a) Discharging capacity vs. cycle number for the bare silicon electrode and the Si–MnOOH electrode at different currents in the voltage range of 0.01–1.5 V; (b) voltage profiles of the bare silicon electrode and the Si–MnOOH electrode at current densities of 0.1, 0.5, 1, 4, 8, 12, 16 and 20 A g 1; (c) AC impedance of the bare silicon electrode and the Si–MnOOH electrode after 100 cycles.
discharged to around 1600 mA h g 1. The voltage–capacity profile in Fig. 3b more clearly shows the capacity/energy advantage of Si–MnOOH over Si. For Si–MnOOH, the capacity only dropped from 3197 mA h g 1 at 0.1 A g 1 to 2800 mA h g 1 at 1 A g 1, while the voltage gap between charging and discharging remained almost constant. Even at a current of 12 A g 1, a decent capacity of around 1200 mA h g 1 was delivered. It is worth noting that the Si–MnOOH electrode presented an appreciable capacity of 700 mA h g 1 at 20 A g 1. To our knowledge, such capacity has seldom been reported on silicon-based materials in the ultrafast charge/discharge mode. Actually, even on a ‘‘super thick’’ Si–MnOOH electrode (mass loading is 3 mg cm 2, the ratio of active material : acetylene black : PAA binder is 6 : 3 : 1, specific area capacity at 1 A g 1 is 4.5 mA h cm 2), the improvement in rate capability is still very significant (Fig. S5 in ESI†), however, the stable cycling will require more work on the binder because of much more serious electrode peeling during electrode fabrication and electrode cycling.
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As the performance is obtained on the Si–MnOOH composite prepared by a simple ball-milling, further enhanced capacity/ energy performance can be reasonably expected if architecture design or component optimization can be involved. The AC impedance comparison provides some clues for the better electrochemical performance of Si–MnOOH than Si (Fig. 3c). After 100 cycles, impedance spectra were recorded on both electrodes. They all showed two semicircles in the high and high-to-medium frequency range, which represents Li+ diffusion through the SEI and charge-transfer reaction, and can be indexed by RSEI and Rct, respectively, according to the equivalent circuit diagram. The low-frequency Warburg slope represents the semi-infinite diffusion of lithium-ion in the electrodes.24 Comparing the impedance diagrams, we can see that the Si–MnOOH electrode has smaller semicircles than the Si electrode in the high-frequency and medium-frequency regions, suggesting the smaller RSEI and Rct for the Si–MnOOH electrode than the Si electrode. The fitted data in Table S1 (in ESI†) indicate that the Rct value of Si–MnOOH is much smaller than that of the Si electrode, and the result reveals much better reaction kinetics of Si–MnOOH than Si. The very close Warburg factor (s) of Si and Si–MnOOH electrodes suggests a similar Li+ diffusion coefficient of these two electrodes,25 and it is reasonable because Si and Si–MnOOH have very similar particle size and essentially the same structure (the very low proportion of MnOOH in the Si–MnOOH composite will not significantly affect the structure of bulk materials). We think that the impedance results explain the excellent cycling stability and rate capability of the Si–MnOOH composite and may imply the ‘‘catalyzing’’ effect of g-MnOOH on the alloying/de-alloying reaction of the Si anode; more work is still in progress on this point. In conclusion, the g-MnOOH nanorod was prepared through a hydrothermal method, and then ball-milled with Si to form the Si–MnOOH composite. The as-prepared Si–MnOOH composite showed a high capacity of about 3900 mA h g 1 and very stable cycling. Additionally, the composite exhibited an excellent rate capability, such as 1200 mA h g 1 at 12 A g 1, and 700 mA h g 1 at 20 A g 1. The result provides a new strategy to enhance the electrochemical performance of the Si anode, and the facile preparation offers a new way to obtain mass-producible Si anode material with perfect Li-ion storage properties. The authors would like to express their sincere thanks to the National Natural Science Foundation of China (No. 21073138, 21273168, 21233004) for the financial support.
Notes and references 1 J. X. Song, M. J. Zhou, R. Yi, T. Xu, M. L. Gordin, D. H. Tang, Z. X. Yu, M. Regula and D. H. Wang, Adv. Funct. Mater., 2014, 24, 5904–5910. 2 J. H. Ryu, J. W. Kim, Y. E. Sung and S. M. Oh, Electrochem. Solid-State Lett., 2004, 7, A306–A309. 3 X. Yang, Z. Wen, X. Xu, Z. Gu and S. Huang, Electrochem. Solid-State Lett., 2007, 10, A52–A55. 4 M. Y. Nie, D. P. Abraham, Y. J. Chen, A. Bose and B. L. Lucht, J. Phys. Chem. C, 2013, 117, 13403–13412. 5 Y. Liu, X. Guo, J. F. Li, Q. L. Lv, T. Y. Ma, W. T. Zhu and X. P. Qiu, J. Mater. Chem. A, 2013, 1, 14075–14079.
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Communication 6 N. Du, X. Fan, J. X. Yu, H. Zhang and D. R. Yang, Electrochem. Commun., 2011, 13, 1443–1446. 7 R. Teki, M. K. Datta, R. Krishnan, T. C. Parker, T. M. Lu, P. N. Kumta and N. Koratkar, Small, 2009, 5, 2236–2242. 8 H. Zhong, H. Zhan and Y. H. Zhou, J. Power Sources, 2014, 262, 10–14. 9 N. Liu, H. Wu, M. T. McDowell, Y. Yao, C. M. Wang and Y. Cui, Nano Lett., 2012, 12, 3315–3321. 10 J. C. Guo and C. S. Wang, Chem. Commun., 2010, 46, 1428–1430. 11 I. Kovalenko, B. Zdyrko, A. Magasinski, B. Hertzberg, Z. Milicev, R. Burtovyy, I. Luzinov and G. Yushin, Science, 2011, 334, 75–79. 12 Y. M. Lin, K. C. Klavetter, P. R. Abel, N. C. Davy, J. L. Snider, A. Heller and C. B. Mullins, Chem. Commun., 2012, 48, 7268–7270. 13 V. M. B. Crisostomo, J. K. Ngala, S. Alia, A. Dobley, C. Morein, C. H. Chen, X. F. Shen and S. L. Sui, Chem. Mater., 2007, 19, 1832–1839. 14 G. Tao, K. Frank and N. Reinhard, Inorg. Chem., 2009, 48, 6242–6250. 15 L. L. Zhang, X. B. Zhang, Z. L. Wang, J. J. Xu, D. Xua and L. M. Wang, Chem. Commun., 2012, 48, 7598–7600.
Chem. Commun.
ChemComm 16 Y. Xia, Z. Xiao, X. Dou, H. Huang, X. H. Lu, R. J. Yan, Y. P. Gan, W. J. Zhu, J. P. Tu, W. K. Zhang and X. Y. Tao, ACS Nano, 2013, 7, 7083–7092. 17 G. L. Xu, Y. F. Xu, H. Sun, F. Fu, X. M. Zheng, L. Huang, J. T. Li, S. H. Yang and S. G. Sun, Chem. Commun., 2012, 48, 8502–8504. 18 S. B. Ni, X. H. Lv, T. Li, X. L. Yang and L. L. Zhang, J. Mater. Chem. A, 2013, 1, 1544–1547. 19 X. H. Huang, J. P. Tu, X. H. Xia, X. L. Wang and J. Y. Xiang, Electrochem. Commun., 2008, 10, 1288–1290. 20 J. C. Guo, A. Sun and C. S. Wang, Electrochem. Commun., 2010, 12, 981–984. 21 L. C. Yang, Q. S. Gao, Y. H. Zhang, Y. Tang and Y. P. Wu, Electrochem. Commun., 2008, 10, 118–122. 22 Y. F. Shi, B. K. Guo, S. A. Corr, Q. H. Shi, Y. S. Hu, K. R. Heier, L. Q. Chen, R. Seshadri and G. D. Stucky, Nano Lett., 2009, 9, 4215–4220. 23 J. Li and J. R. Dahn, J. Electrochem. Soc., 2007, 154, A156–A161. 24 J. Liu and A. Manthiram, Chem. Mater., 2009, 21, 1695–1707. 25 H. Liu, C. Li, H. P. Zhang, L. J. Fu, Y. P. Wu and H. Q. Wu, J. Power Sources, 2006, 159, 717–720.
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